1. Field of the Invention
The present invention relates to a grain oriented electrical steel sheet, particularly to a grain oriented electrical steel sheet having tenacious adhesion to tension coating films and having a very low iron loss. The invention further relates to a novel method for producing the same.
2. Description of the Related Art
Grain oriented electrical steel sheets that contain Si, and which have crystal grains that align to the (110) [001] or (100) [001] orientations are widely used as iron core materials. They are often used in the commercial frequency region. The steel sheets have excellent soft magnetic characteristics.
It is important for this kind of steel sheet to have a low iron loss W.sub.17/50 when it is magnetized to 1.7T at a frequency of 50 Hz or 60 Hz.
Many methods of making electrical steel sheets are known in the art. These include enhancing electrical resistance by causing Si to be present, decreasing the thickness of the steel sheet, lowering the eddy current loss by diminishing the crystal grain size, and lowering the hysteresis loss by aligning effective crystal orientation. It is additionally known to apply tension material to the steel sheet surface.
Of the methods cited above, the presence of Si may cause an increase of the iron core value, because of decreased saturation magnetic flux density, when the Si content becomes too large. Decreasing sheet thickness may result in extremely high production cost. Although a magnetic permeability of 1.96T or 1.97T can be obtained at a magnetic flux density B.sub.8 by aligning the crystal grain orientation, thereby reducing the iron loss, further improvements are needed, but are almost beyond expectation.
Technologies have been developed involving artificial refinement of the magnetic domain width on the steel to reduce iron loss. These technologies include introducing local strains by irradiating the steel sheet surface with a plasma jet or a laser beam, or forming grooves on the steel sheet surface. Although the iron loss has been reduced by applying these technologies, the extent of the reducing is limited.
Smoothing the surface of the electrical steel sheet, to reduce pinning sites that inhibit movement of magnetic domain walls in the vicinity of the steel sheet surface during the magnetization process, has been disclosed. For example, Japanese Examined Patent Publication No. 52-24499 discloses a method for removing surface products by pickling with an acid after final annealing, followed by mirror-finishing the surface by chemical or electrolytic polishing to reduce the roughness of the interface between the steel sheet surface and the non-metallic coating film. Japanese Unexamined Patent Publication No. 5-43943 discloses subjecting the steel sheet to thermal etching in H.sub.2 gas at a temperature of 1000 to 1200.degree. C. after removing forsterite films.
Japanese Examined Patent Publication Nos. 4-9041, 5-87597 and 6-37694 disclose reducing the iron loss by enhancement treatment of crystal grain orientation in order to cause crystal grains of a specific orientation to remain on the metal surface, thereby reducing the iron loss of the material.
In order to obtain reduced iron loss by use of any of the methods set forth above, it is inevitable to apply a strong tension film to the surface of the steel sheet. When no tension film is present, the steel sheet surface becomes so smooth that enlargement of the magnetic domain width is accelerated. This results in deterioration of the iron loss. Therefore, it is necessary that the tension coating film remains on the steel sheet surface.
In the currently available technology, to attain the objectives set forth above, a film comprising a substance having a smaller heat expansion coefficient than the steel sheet is formed. For example, a film mainly composed of forsterite is formed in the so-called final annealing step by reacting oxides on the steel sheet surface with an annealing separator coated thereon, followed by applying a top coat (a tension coating film with a low heat expansion coefficient) as a tension-endowing type insulation film on the forsterite film.
This tension-endowing type insulation film is mostly formed by coating and baking a treatment liquid mainly composed of a phosphate salt of Al and alkali earth metals, colloidal silica and chromic anhydride or chromic acid salts, endowing the steel sheet with a tension at room temperature by taking advantage of the heat expansion difference between the iron substrate and an insulation film such as an inorganic coating film represented by colloidal silica having a smaller coefficient of heat expansion than that of the steel sheet. Representative methods for forming the insulation films are described in the art disclosed, for example, in Japanese Examined Patent Publication Nos. 53-28375 and 56-52117.
However, the foregoing methods have drawbacks caused by the fact that adhesion of the tension-endowing type coating film is poor. In other words, the coating film with a larger tension-endowing effect needs a stronger adhesive force, because this film might be peeled off if the adhesive force of the substrate is not strong enough to hold the coating film when the tension-endowing type coating film is directly applied on the metal substrate without forsterite film as a result of a surface smoothing treatment such as a surface mirror finish, resulting in a very poor adhesive property of the substrate. Consequently, it is a crucial problem to make the technology for magnetically smoothing the surface of the electrical steel sheet compatible with the iron loss reducing technology using a tension-endowing type insulation film. This has become a major problem in the art.
The technologies for magnetically smoothing the surface of the iron substrate of the electrical steel sheet and for endowing the steel sheet with the tension coating film will be summarized hereinafter.
Japanese Examined Patent Publication No. 56-4150 discloses a method for smoothing the steel sheet surface to a mean surface roughness Ra of 0.4 .mu.m or less by applying chemical polishing or electrolytic polishing, followed by depositing a ceramic thin film thereon. This is done by chemical vapor deposition or vacuum deposition. While a large tension effect can be generated by this method due to the differences of heat expansion coefficients, since the ceramic coating film has a substantially smaller heat expansion coefficient as compared to that of the iron substrate, adhesion between the iron substrate and the coating film becomes quite a problem. Further, this method is not suitable for industrial production owing to slow deposition of the coating film.
Japanese Examined Patent Publication No. 63-54767 discloses a method for forming ceramic films made of, for example, nitrides or carbides by ion plating or ion implantation. Japanese Examined Patent Publication No. 2-243770 discloses directly forming a tension endowing type insulation film on the steel sheet surface by depositing a ceramic coating film, sing a so-called sol-gel method. However, forming the ceramic coating film by deposition as disclosed in Japanese Examined Patent Publication No. 63-54767 requires a high production cost along with being difficult to attain uniform film thickness in mass-treatment of large area films. Although film formation by baking is possible in the sol-gel method according to Japanese Examined Patent Publication No. 2-243770, however, it is difficult to form an intact film with a thickness of 0.5 .mu.m or more, the film lacking the benefit of any large tension-endowing effect. In addition, the film has such poor adhesiveness to the steel sheet that the desired iron loss improvement effect cannot be obtained.
Japanese Unexamined Patent Publication No. 62-103374 discloses a method for depositing a mixed thin film of the iron substrate with a variety of oxides, borates, phosphates and sulfides on a steel sheet surface smoothed by polishing, on which a baked layer of an insulation film is formed. While this method provides excellent adhesion of the steel sheet to the baked layer of insulation film, improvement of magnetic characteristics cannot be realized since the smoothing effect of the steel sheet into a mirror finish is lost due to the presence of the mixed thin layer with the iron substrate.
Japanese Unexamined Patent Publication No. 6-184762 disclosed a method for coating and baking a coating solution or forming a tension endowing coating film after allowing SiO.sub.2 to deposit. However, the method for depositing the SiO.sub.2 thin film results in such a poor tension effect that the iron loss improvement becomes insufficient.
Japanese Unexamined Patent Publication No. 7-173641 proposes a grain oriented electrical steel sheet provided with a metallic coating film whose linear heat expansion coefficient is decreased to 3.times.10.sup.-6 K.sup.-1 or lower by applying heat treatment on the steel sheet surface. However, little iron loss reduction can be achieved when the interface between the surface of the iron substrate of the steel sheet and the metallic coating film has substantial roughness. Further, it is impossible to obtain the desired effect because the metallic coating film layer is peeled off by the heat treatment when the interface is smooth.
Japanese Unexamined Patent Publication No. 3-294468 discloses forming a silicide coating film with a low pressure plasma deposition after applying a metallic plating on the smoothed surface of the iron substrate. However, the adhesion between the metallic plating film and the plasma deposit silicide film is not sufficient to achieve the desired magnetic characteristics.
The foregoing Japanese Unexamined Patent Publication No. 52-24499 further discloses a method for coating and baking the coating solution to form a tension film by mirror finishing the surface of the iron substrate of the steel sheet, followed by plating a metallic thin film on the sheet. While this process can suppress reduction of magnetization due to degradation of the steel sheet surface, the insulation film after metallic plating is prone to being peeled off by baking or, even if peeling could be avoided, a large degree of iron loss reduction could not be achieved because the insulation film was composed of a non-tension insulation film of an ordinary phosphate origin.
Although an iron loss reduction could be expected if the insulation film is of a tension-endowing type, such means are practically impossible because the coating film has only weak adhesion to the plating surface.
As hitherto described, the trend of technical development for reducing the iron loss of grain oriented electrical steel sheet is directed to the concept of forming a tension coating film after smoothing the surface of the iron substrate of the steel sheet, or after applying a enhancement treatment of crystal grain orientation. However, the tension coating film exerts so strong a tension on the steel sheet surface that the interface between the steel sheet surface and tension coating film suffers a strong shearing stress that naturally tends to peel off the coating film. Consequently, the desired tension is not applied and significant iron loss reduction cannot be attained.
While it may be readily presumed that enhancing the interface roughness between the surface of the iron substrate of the steel sheet and the tension coating film is effective for solving such problems, smoothness of the steel sheet surface is lost by this method and this negates the favorable iron loss characteristics.
Although adhesive properties of the tension coating film are somewhat improved by enhancement treatment of crystal grain orientation as compared with a smoothing treatment, yet the resulting adhesive properties are so far from the desired adhesion that the iron loss cannot be sufficiently reduced, since the desired tension effect is not fully imparted to the steel sheet.